We have discovered that the sympathetic preganglionic neurons (SPN's) of rats as old as 21 days (weaning) exhibit an extremely immature morphology. This observation suggests that although many rudimentary autonomic reflexes may be functional at this age, significant supraspinal control over SPN's develops between weaning and somatic maturity. This post weaning ontogeny would include the activation, in normal rats (NR's), of those excitatory and inhibitory systems which mediate an appropriate level of sympathetic tone during the socially complex post weaning period. This post weaning period of sympathetic morphogenesis also includes, in spontaneously hypertensive rats (SHR's), the onset of those factors which result in elevated sympathetic tone and abnormally high arterial pressure. This project will compare, in NR's and SHR's, the physiological and morphological development of SPN's and the supraspinal systems which control the activity of these neurons. Ample evidence now exists to implicate the sympathoadrenal system in spontaneous hypertension in rats. However, so little is known about the determinants of sympathetic preganglionic activity and about the ontogeny of those systems which normally maintain an appropriate balance of sympathetic activity that etiological mechanisms have been difficult to formulate. This project will answer the following questions. 1. Do morphogenic differences (correlated with severity of hypertension) exist between the SPN's of NR's and SHR's? 2. Do sympathoexcitatory and sympathoinhibitory systems develop at the same rates in NR's and SHR's? 3. Do the integrative properties of the SPN's of NR's and SHR's develop similarly? 4. Does neonatal destruction of pathways from supraspinal vasomotor systems modify the development of arterial pressure regulation and the physiological and morphological properties of SPN's in NR's and SHR's?